1. Field Of The Invention
The present invention relates to carbonless copying systems and in particular to microcapsules which are useful in connection with such systems and which comprise minute discrete droplets of liquid fill material including an initially colorless chemically reactive color forming dye precursor and a carrier therefor encapsulated within individual, rupturable, generally continuous shells.
2. Description Of The Prior Art
Impact or pressure sensitive carbonless transfer papers have recently come into wide usage in the U.S. and throughout the world. Ordinarily, such papers are printed and collated into manifolded sets capable of producing multiple copies. In this connection, pressure applied to the top sheet causes a corresponding mark on each of the other sheets of the set.
The top sheet of paper, upon which the impact or pressure is immediately applied, ordinarily has its back surface coated with microscopic capsules containing one of the reactive ingredients which interreact to produce a mark. A receiver sheet, placed in contact with such back face of the top sheet has its front surface coated with a material having a component which is reactive with the contents of the capsule so that when capsules are ruptured upon impact by stylus or machine key, the initially colorless or substantially colorless contents of the ruptured capsules react with a co-reactant therefor on the receiver sheet and a mark forms on the latter corresponding to the mark impressed by the stylus or machine key.
In the art, impact transfer papers are designated by the terms CB, CFB and CF, which stand respectively for "coated back", "coated front and back", and "coated front". Thus, the CB sheet is usually the top sheet and the one on which the impact impression is directly made; the CFB sheets are the intermediate sheets, each of which have a mark formed on the front surface thereof and each of which also transmits the contents of the ruptured capsules from its back surface to the front surface of the next succeeding sheet; and the CF sheet is the last sheet and is only coated on its front surface to have an image formed thereon. The CF sheet is not normally coated on its back surface as no further transfer is desired.
While it is customary to coat the capsules on the back surface and to coat the co-reactant for the capsule contents on the front surface of each sheet, this procedure could be reversed if desired. Further, with some systems, coatings need not be used at all and the co-reactive ingredients may be carried in the sheets themselves, or one may be carried in one of the sheets and the other may be carried as a surface coating. Further, the co-reactive materials may each be microencapsulated. Patents illustrative of many of the various kinds of systems which may incorporate such co-reactive ingredients and which may be used in the production of manifolded transfer papers include, for example, U.S. Pat. No. 2,299,694 to Green, U.S. Pat. No. 2,712,507 to Green, U.S. Pat. No. 3,016,308 to Macaulay, U.S. Pat. No. 3,429,827 to Ruus and U.S. Pat. No. 3,720,534 to Macaulay et al.
The most common variety of carbonless impact transfer paper, and the type with which the present invention is utilized, is the type illustrated, for example, in Green U.S. Pat. No. 2,712,507 and Macaulay U.S. Pat. No. 3,016,308 wherein microscopic capsules containing a liquid fill comprising a solution of an initially colorless chemically reactive color forming dye precursor are coated on the back surface of the sheet, and a dry coating of a coreactant chemical for the dye precursor is coated on the front surface of a receiving sheet.
Many color precursors useful in connection with carbonless copying systems are known to those skilled in the art to which the present invention pertains. For example, specific reference is made to the color precursors mentioned in the patent to Phillips, Jr. et al, U.S. Pat. No. 3,455,721 and particularly to those listed in the paragraph bridging columns 5 and 6 thereof. These materials are capable of reacting with a CF coating containing an acidic material such as an acidleached bentonite-type clay or the acid-reactant organic polymeric material disclosed in the Phillips, Jr. et al U.S. Pat. No. 3,455,721 patent. Many of the color precursors disclosed in the U.S. Pat. No. 3,455,721 patent referred to above are capable of undergoing an acid-base type reaction with an acidic material. Other previously known color precursors are the spiro-dipyran compounds disclosed in the patent to Harbort, U.S. Pat. No. 3,293,060 with specific reference being made to the disclosure of the U.S. Pat. No. 3,293,060 patent extending from column 11, line 32 through column 12, line 21. The color precursors of Harbort, as well as the color precursors of Phillips, Jr. et al are initially colorless and are capable of becoming highly colored when brought into contact with an acidic layer such as an acid-leached bentonite-type clay or an acid-reacting polymeric material, or the like.
Generally speaking, color precursor materials of the type disclosed by Phillips, Jr. et al U.S. Pat. No. 3,455,721 and by Harbort U.S. Pat. No. 3,293,060 are dissolved in a solvent and the solution is encapsulated in accordance with the procedures and processes described and disclosed in U.S. Pat. No. 3,061,308 to Macaulay, U.S. Pat. No. 2,712,507 to Green, U.S. Pat. No. 3,429,827 to Ruus and U.S. Pat. No. 3,578,605 to Baxter. In this connection, it should be mentioned that the present invention is particularly useful in connection with microcapsules of the type disclosed by Ruus U.S. Pat. No. 3,429,827 which are produced by an interfacial polycondensation procedure.
Solvents known to be useful in connection with dissolving color precursors include chlorinated biphenyls, vegetable oils (castor oil, coconut oil, cotton seed oil, etc.), esters (dibutyl adipate dibutl phthalate, butyl benzyl adipate, benzyl octyl adipate, tricresyl phosphate, trioctyl phosphate, etc.), petroleum derivatives (petroleum spirits, kerosene, mineral oils, etc.), aromatic solvents (benzene, toluene, etc.), silicone oils, or combinations of the foregoing. Particularly useful are the alkylated naphthalene solvents disclosed in U.S. Pat. No. 3,806,463 to Konishi et al.
In the color forming systems outlined above, as will be appreciated by those skilled in the art, the color precursors are conventionally contained in pressure rupturable microcapsules which are included in the back coatings of the sheets of carbonless copying manifolded sets. Further, it will be appreciated that the acidic coatings are generally utilized as front coatings with the color precursor material in a solvent therefor being transferred from an adjacent back coating to the acidic layer front coating upon rupture of the capsules which contain the color precursor material.
Although microcapsules have been extensively used in connection with carbonless copying systems in the past, one particular shortcoming, which has continued to detract from such systems, both from an economical and from an operational point of view, is the inadvertent or unintentional development of color on the CF coatings. Free colorless dye precursor has often been present in CB coatings in the past due to limitations of the encapsulation procedure, or due to accidental capsule rupture which often occurs during handling, coating processes, printing processes, etc. This free precursor often causes discoloration by contacting the CF ingredients through the base paper in the CFB sheets and from sheet to sheet in a manifolded set or form. This discoloration, which is sometimes referred to as blush, offset, bluing, etc., is highly objectionable and undesirable in a copying or imaging system.
High surface area fillers such as Syloids (synthetic silicas) have been utilized in admixture with the microcapsules in CB coatings to prevent blush with some success. These fillers absorb free dyes or solvents or both and substantially reduce the quantity of dye material which is free to be transferred to an adjacent CF coating. However, the inclusion of such additives in CB coatings increases the cost of the latter and often such additives operate to reduce image intensity. The foregoing concepts as well as other prior art procedures directed to alleviating the problem of inadvertent CF discoloration in carbonless copying systems are disclosed in U.S. Pat. No. 3,617,334 to Brockett et al; U.S. Pat. No. 3,481,759 to Ostlie; and U.S. 3,625,736 to Matsukawa et al. Also note British Pat. Nos. 1,232,347 and 1,252,858 which disclose the intermixture of finely divided particles of starch or starch derivatives with microcapsules for the purpose of reducing stain-formation during the processing of pressure sensitive recording paper. British Pat. No. 1,252,858 also discloses the use of hard, inert beads (such as fine glass beads) and short cellulose fibers or floc as a stilt material to guard against unintended capsule rupture and the consequent development of coloration and smudging from frictional pressures encountered in the handling and use of carbonless copying papers.